Overview of Disk Galaxies in the Context of Other Galaxies

Gaia ITNG2013 School, Tenerife Ken Freeman, Lecture 1: overview M83 B K September 2013

My lectures are on stellar populations: compositions, kinematics and morphology: Lecture 1: overview of the structure of our Galaxy in the context of other galaxies. the basic components, dark matter properties, general ideas about galaxy assembly

Disk Galaxies The disks are flat. They are supported by rotation and usually show spiral structure delineated by gas and young stars. The random motions of gas and stars are small, sothe stellar and gas orbits in the disk lie almost in the plane of the disk The stars of the disk have a large range of ages Disk galaxies may or may not have a central bulge and a central bar.

Our Galaxy, the Milky Way is near the upper end of the mass range of spiral galaxies M* ~ 6.1010 M Mtotal ~ 1.5 1012M

Brighter spirals rotate more rapidly. This is the Tully-Fisher (1977) law. Vf is the rotational velocity in km s-1 This example shows the baryonic TF law Mb vs Vf The MW(+) lies on the TF relation, near the massive end McGaugh 2011

The nearby spiral galaxy M83 in blue light (L) and at 2.2 (R) M83 is similar in size and morphology to the MW. The blue image shows young star-forming regions and is affected by dust obscuration. The NIR image shows mainly the old stars and is unaffected by dust. Note how clearly the central bar can be seen.in the NIR image. The MW has a similar bar.

MW The size of the bulge ranges from very large to vanishingly small. Note how some bulges are more or less round, and others have a boxy shape like the MW. Bulges are a common but not essential feature of spirals

Two dynamical questions: • What keeps the disk in equilibrium Most of the kinetic energy is in the rotation in the radial direction, gravity provides the radial acceleration needed for the near-circular motion of the stars and gas in the vertical direction, gravity is balanced by the vertical pressure gradient associated with the random vertical motions of the disk stars. • Where do the bars of barred spirals come from ? Believed to come from bar-like (m=2) instabilities of rotationally supported disks.

The rotation curves of spirals (usually measured for the neutral hydrogen) reveals their massive dark halos. We can estimate the DM content of the Milky Way out to R ~ 200 kpc +. The mass fraction in DM is about 97% for the Milky Way. NGC 3198 Rotation at large radii is much faster than can be understood from the gravitational field of the stars and gas alone.

The surface brightness distribution of disk galaxies. The disks usually follow an exponential surface brightness distribution in radius R : at least for a few scalelengths (h) Io is the central surface brightness, typically around h is the scale length: ~ 4 kpc for a large galaxy like the MW ~ 1.5 kpc for a smaller galaxy like M33 The ratio of stars/gas varies : for the Milky Way. the stars 95%, gas 5% of the visible matter. Dark/(visible baryonic mass) ratio is about 30, compared with the cosmic ratio of about 6.

M33 - outer disk truncated, very smooth structure The Exponential Disk I(R) is exponential for about 5 scalelengths, and then steepens or truncates abruptly Ferguson et al 2003

NGC 300 - exponential disk goes for at least 10 scale-lengths without truncation NGC 300 is otherwise almost identical to M33, but its I(R) stays exponential for at least 10 scalelengths. Bland-Hawthorn et al 2005

Erwin et al (2005) classification of surface brightness profile morphologies: disk profiles show three classes of shape mostly interacting

Disk truncation is not yet well understood. Some disks like NGC 300 are not truncated at all, down to very faint surface brightness levels. The type III (anti-truncations) may be the outcome of interactions - disk material is moved out to larger radii.

The type II truncations may be associated with • the star formation threshold • angular momentum redistribution by bars and spiral waves • the hierarchical accretion process • bombardment by dark matter subhalos (de Jong et al 2007) Roskar et al (2008) - SPG simulation of disk formation from cooling gas in an isolated dark halo : includes star formation and feedback. The break is seeded by rapid radial decrease in surface density of cool gas : break forms within 1 Gyr and gradually moves outwards as the disk grows. The outer exponential is fed by secularly redistributed stars from inner regions (Sellwood & Binney 2002) so its stars are relatively old.

stellar surface density gas surface density star formation rate mean stellar age Roskar et al (2008)

The vertical structure of disks The vertical structure is also close to exponential  = o exp (z/hz) so overall the disk is close to a double exponential  = o exp (R/hR +z/hz) Some authors prefer a vertical sech2(z/zo) law which is nearly exponential for z >> zo. Historically this was favored because  = osech2 (z/zo) is a selfconsistent solution for a vertically isothermal sheet (i.e. the dispersion z is independent of z).

K surface brightess de Grijs et al 1997 z The vertical structure of disks is directly associated with their star formation history and dynamical history: scattering, accretion, heating, warping … these processes generate a vertical scale height hz for the old thin disk that is usually about 200-300 pc.

Sb Sc Radial gradient of disk scaleheight For late-type galaxies, the scaleheight hz is almost independent of radius - constraint on heating mechanism de Grijs & Peletier 1997

The constant observed scaleheight for late type disks is interesting ... • The scaleheight hz is related locally to the surface density • of the disk and to the local vertical velocity dispersion z via hz ~ z2 /  , so constant scaleheight means that the local disk heating is tightly related to the local surface density of disk matter. See Lacey & Fall (1983) for discussion based on relation between heating and star formation rate.

R (kpc) For constant scaleheight, expect exponential decrease in the disk’s velocity dispersion with radius. Lewis & Freeman 1989

Spiral galaxies are believed to form as baryons settle in the potential of their dark halo. They settle to a flat disk before forming stars, roughly conserving the baryon angular momentum. The outcome is a disk in near centrifugal equilibrium. The details of the baryon settling process are not well understood. It is probably still going on. This is a busy topic right now.

Overview of Our Galaxy Because we lie within our Galaxy, much more detail is known about its structure than for most other galaxies Schematic picture of our Galaxy, showing bulge, thin disk, thick disk, stellar halo and dark halo

Each one of these components has something to tell us about its formation history Our task is to understand how the formation and evolution of the Milky Way took place: how does it compare with the predictions of CDM simulations ?

The thin disk is metal-rich and covers a wide age range The other stellar components are all relatively old (note similarity of [Fe/H] range for thick disk and globular clusters)

Total mass ~ 1-2 x 1012 M : Wilkinson & Evans (1999), Sakamoto et al (2003), Deason et al (2012) ... Stellar mass in bulge 1 x 1010 M disk 5 x 1010 M stellar halo 1 x 109 M Ages of components: globular clusters ~ 10-12 Gyr thick disk : > 10 Gyr thin disk : star formation started about 10 Gyr ago and star formation in the disk has continued at a more or less constant rate to the present time

How did the Galaxy come to be like this ? To study the formation of galaxies observationally, we have a choice ... we can observe distant galaxies at high redshift : we see the galaxies directly as they were long ago, at various stages of their formation and evolution but not much detail can be measured about their chemical properties and motions of their stars so we cannot follow the evolution of any individual galaxy

or we can recognise that themain structures of our Galaxy formed long ago at high redshift. the halo formed at z > 4 the disk formed at z ~ 2 We can study the motions and chemical properties of stars in our Galaxy at a level of detail that is impossible for other galaxies, and probe into the formation epoch of the Galaxy. This is near-field cosmology

The ages of the oldest stars in the Galaxy are similar to the lookback time for the most distant galaxies Both give clues to the sequence of events that led to the formation of galaxies like the Milky Way

MOVIE Start by showing a numerical simulation of galaxy formation. The simulation summarizes our current view of how a disk galaxy like the Milky Way came together from dark matter and baryons, through the merging of smaller objects in the cosmological hierarchy. • much dynamical and chemical evolution • halo formation starts at high z • dissipative formation of the disk

Simulation of galaxy formation • cool gas • warm gas • hot gas

Movie synopsis •z ~ 13 :star formation begins - drives gas out of the protogalactic dark matter mini-halos. Surviving stars will become part of the stellar halo - the oldest stars in the Galaxy • z ~ 3 :galaxy is partly assembled - surrounded by hot gas which is cooling out to form the disk - rapid chemical evolution occurs from z ~ 3 to z ~ 1 in most spirals • z ~ 2 :large lumps are falling in - now have a well defined rotating disk galaxy. You saw the evolution of the baryons. There is about 10 x more dark matter in a dark halo, underlying what you saw: The dark halo was built up from mergers of smaller sub-halos Saw spiral structure developing in the gas Merging of galaxies is still going on now

The movie showed the formation and evolution of a large spiral in a CDM simulation. What does each component of the Milky Way contribute to our understanding of the formation and evolution of disk galaxies in the CDM context ? Living inside the Milky Way has advantages and disadvantages. The Milky Way is very good for assessing some issues and not so good for others

What are the issues with galaxy formation in CDM in the context of what our Galaxy can contribute towards understanding these issues? • Structure of the inner dark halo - core or cusp • Number of predicted satellites • Forming disks with small bulges in CDM • Active accretion history • Baryonic angular momentum

What are the issues with galaxy formation in CDM ? • Number of predicted satellites From simulations, we would expect a galaxy like the Milky Way to have ~ 500 satellites with bound masses > 108 M. These are not seen optically or in HI. New very faint satellites are being discovered but unlikely to find 500 Are there large numbers of dark satellites ? Are some (or all) globular clusters the nuclei of accreted fragments ? B. Moore et al

What are the issues with galaxy formation in CDM ? • Forming disks with small or no bulges in CDM It is currently difficult for CDM to generate galaxies with small or no bulges. Understanding how the bulge of the Galaxy formed is important for this problem. Some recent progress in simulations - requires high feedback and high star formation threshold (eg Brook et al 2011)

Small bulges are thought to be generated by instability processes within disks, rather than by merger activity. If that is correct, then an even larger fraction of disk galaxies were born without bulges, and the problem of forming pure disk systems becomes even more evident (more in lecture on the bulge)

Sgr NGC 5907 What are the issues with galaxy formation in CDM ? • CDM predicts an active ongoing accretion history, leaving debris of accreted satellites in the stellar disk and halo. (The first stars probably came from small dense accreted systems which formed before the Milky Way itself). A very active accretion history may be inconsistent with the presence of a dominant thin disk. Epoch of last major merger is particularly important for disk survival. We are uniquely able in the Milky Way to evaluate accretion history of a large spiral and measure the distribution of its first stars Chou APOD

• Baryon acquisition is needed to fuel ongoing star formation, which would exhaust the current gas supply on a timescale ~ few Gyr. How is this happening ? Is it related to the accretion history, high velocity HI clouds, the galactic warp ? Is it gas that was previously ejected from the disk ? Milky Way is potentially well suited to investigate baryon acquisition.

Galaxy Mergers Mergers of galaxies are important in the early universe, as galaxies are assembled through a heirarchy of mergers. They remain important at the present time for transforming disk galaxies into giant ellipticals. Mergers stimulate star formation and starbursts, and are significant in contributing to chemical evolution of galaxies and to enrichment of the circumgalactic gaseous medium. Accretion of small galaxies continues to the present time and contributes to the formation of the metal-poor halo of our Galaxy. We will look later in more detail at the dynamics of merging via dynamical friction and tidal disruption.

Two disk galaxies interact tidally and merge. Merging stimulates star formation and disrupts the galaxies. This is NGC 4038/ 9 - note the long tidal arms . The end product of the merger is often an elliptical galaxy.

NGC 1316: a bright late merger remnant in the Fornax cluster. It may end up looking like the Sombrero galaxy (McNeil et al 2012), with a large bulge and a late-forming disk

Reconstructing galaxy formation We would like to reconstruct the whole process of galaxy formation, as the Galaxy comes together from the CDM hierarchy. What do we mean by the reconstruction of Galaxy formation ? We want to understand the sequence of events that led to the Milky Way as it is now.Ideally, we would like to tag or associate the visible components of the Galaxy to partsof the proto-galactic hierarchy : i.e. to the baryon reservoir which fueled the stars in the Galaxy. This seems too difficult. In the process of galaxy formation and evolution from the CDM hierarchy, a lot of information about the proto-galactic hierarchy is lost. Now discuss how information is lost during galaxy formation and evolution.

Epochs when information about the proto-hierarchy is lost: • As dark matter virialises • As baryons dissipate within the dark halo to form the disk and bulge • As the disk restructures to form the bulge (if that is the way it formed) • Subsequent accretion of objects from the environment : information is lost, though some traces remain. • During the evolution of the stellar disk, as orbits are scattered by dynamical processes - resonances, molecular clouds, spiral arms … At each epoch, some information remains: what does the Galaxy remember ? What can we hope to discover with Galactic Archaeology ?

Accretion is important for building the stellar halo, but not clear yet how much of the halo comes from discrete accreted objects (debris of star formation at high z) versus star formation during the baryonic collapse of the Galaxy At one extreme, simulations of pure dissipative collapse (eg Samland et al 2003) suggest that the halo may have formed mainly through a lumpy collapse, with only ~ 10% of its stars coming from accreted satellites In any case, we can hope to trace the debris of these lumps and accreted satellites from their phase space structure. But we can also use chemical techniques to trace their debris

The chemical composition of galaxies depends on their stellar mass: massive galaxies (M* ~ 1011 M)have mean [Fe/H] ~ 0 while lower mass galaxies have lower mean [Fe/H]. These are the mean values: in each galaxy, there is a wide range of metallicity (e.g. near the sun, the disk stars have metallicities between about -0.5 and +0.5, while the halo stars have metallicities down to -5 (rare)

What generates the chemical evolution of galaxies ? Star formation and subsequent chemical enrichment from • Stellar winds (CNO Na)~ 106 yr • SNII: -elements: Mg, Si, Ca, Ti ; r-process: Eu ~ 107 yr • SNIa: Fe-peak elements Sc-Zn ~ 108-9 yr • AGB stars: s-process Sr, Y, Zr; Ba ~ 108 yr

The mass-metallicity law for galaxies gas poor galaxies gas rich galaxies [Fe/H] - M* [Fe/H] - Mbar Lee, Bell et al 2008

Gas rich and gas poor galaxies follow the same M*-[Fe/H] relation over 9 dex in M* The M*-[Fe/H] relation is defined by the physics of gas-rich galaxies (because they are the ones with active star formation and chemical evolution) The main driver of the M*-Z relation is probably the rate of star formation with mass (lower mass galaxies had a lower rate of star formation) modulated by 1) mass-dependent outflows removing metals 2) variations in stellar IMF 3) evironmental gas removal processes at later times (e.g. stripping in clusters) M82

Overview of Disk Galaxies in the Context of Other Galaxies